National Aeronautics and Administration




Enceladus ’s






Moons of the

www..gov — also called — come in many shapes, , In the realm beyond , has 27 known moons. The SigniFiCAnT DATeS and types. They are generally solid bodies, and few have atmo- inner moons appear to be about half and half . 1610 — Galilei and independently discover . Most of the planetary moons probably formed from the is the most unusual; its chopped-up appearance shows four moons orbiting . Galileo is credited and the moons discs of and circulating around in the early solar the scars of impacts of large rocky bodies. ’s moon are called “Galilean.” system. As of September 2009, have found at least is as big as the dwarf , and backwards 1877 — discovers ’ moons and . 145 moons orbiting planets in our solar system. This number compared with Neptune’s direction of . Neptune has 1969 — Neil is the first of 12 humans to does not include the six moons of the dwarf planets, nor does 13 known moons. walk on the surface of Earth’s Moon. this tally include the tiny satellites that some and Pluto’s large moon, Charon, is about half the of Pluto. Like 1980 — instruments detect signs of surface features other celestial objects. Earth’s Moon, Charon may have formed from resulting beneath the hazy of Saturn’s largest moon, Titan. Of the terrestrial (rocky) planets of the inner solar system, neither from an early of an impactor with Pluto. In 2005, sci- 2000–present — Using improved ground-based , nor has any moons at all, Earth has one, and entists using the Space to study Pluto found the Hubble , and observations, sci- Mars has its two small moons. In the outer solar system, the gas two additional, but very small, moons. The little moons ( and entists have found of new moons in our solar system. (Jupiter, Saturn) and the ice giants (Uranus and Neptune) ) are about two to three times as far from Pluto as Charon Newly discovered moons (as well as other solar system objects) have numerous moons. As these planets grew in the early solar and roughly 5,000 times fainter than Pluto. , another dwarf are given temporary designations until they are confirmed by system, they were able to capture objects with their large gravi- planet even more distant than Pluto, has a small moon of its subsequent observations and receive permanent designations tational fields. own, named . , another , has two from the International Astronomical Union. satellites, Hi’iaka and . Earth’s Moon probably formed when a large body about the size ABoUT THe iMAgeS of Mars collided with Earth, ejecting a lot of material from our FAST FACTS — PlAneTS & SigniFiCAnT MoonS 1 2 1 Selected solar planet into orbit. Debris from the early Earth and the impacting system moons, dis- Mean Radius Mean Radius body accumulated to form the Moon approximately 4.5 billion playing a variety of Planet Moon (km) (mi) 3 ago (the age of the oldest collected lunar rocks). Twelve surface features, are American landed on the Moon during NASA’s Earth Moon 1,737.4 1,079.6 shown at correct rela- program in 1969 to 1972, studying the Moon and bringing 4 6 tive sizes to each other rock samples. Mars Phobos 11.1 6.9 5 and to Earth. Mars Deimos 6.2 3.9 Usually the term “moon” brings to mind a spherical object, like 2 Miranda, a moon of Uranus, has many rugged features: Earth’s Moon. The two , Phobos and Deimos, are Jupiter Io 1,821.6 1,131.9 , grooved structures, ridges, and broken . somewhat different. While both have nearly circular orbits and Jupiter Europa 1,560.8 969.8 3 This false-color of Neptune’s moon Triton shows what travel close to the of the planet’s , they are lumpy Jupiter 2,410 1,498 appear to be volcanic deposits. and dark. Phobos is slowly drawing closer to Mars, and could Jupiter 2,631 1,635 crash into Mars in 40 or 50 million years, or the planet’s 4 This Voyager 1 close-up of Saturn’s moon shows the Saturn Mimas 198.6 123.4 might break Phobos apart, creating a thin ring around Mars. moon’s ancient, cratered surface. Saturn 249.4 154.9 5 A portion of a image of Saturn’s largest moon, Jupiter has 49 known moons (plus 13 awaiting official confirma- Saturn Tethys 529.9 329.3 Titan, showing the complexity of the surface. tion), including the largest moon in the solar system, Ganymede. Saturn Dione 560 348 Many of Jupiter’s outer moons have highly elliptical orbits and Saturn Rhea 764 475 6 Cassini imaged the small when the orbit “backwards” (opposite to the spin of the planet). Saturn, Saturn Titan 2,575 1,600 spacecraft was inbound for Saturn in June 2004. Uranus, and Neptune also have some “irregular” moons, which Saturn 718 446 orbit far from their respective planets. FoR MoRe inFoRMATion Uranus Miranda 235.8 146.5 solarsystem.nasa.gov/planets/profile.cfm?Object=Moons Saturn has 53 known moons (plus 9 awaiting official confirma- Uranus 578.9 359.7 tion). The chunks of ice and rock in Saturn’s rings (and the par- Uranus 584.7 363.3 ticles in the rings of the other outer planets) are not considered Uranus Titania 788.9 490.2 moons, yet embedded in Saturn’s rings are distinct moons or Uranus 761.4 473.1 “.” “Shepherd” moons help keep the rings in line. Sat- Neptune Triton 1,353.4 841 ’s moon Titan, the second largest in the solar system, is the Neptune 170 106 only moon with a thick atmosphere.

LG-2009-09-572-HQ — JPL 400-1344K 09/09